1. Field of the Invention
This invention relates generally to giant magnetoresistive (GMR) magnetic field sensors having a spin valve structure and a “current-perpendicular-to-the-plane” (CPP) configuration. More particularly, it relates to such a sensor that has both an enhanced GMR ratio and low coefficient of magnetostriction.
2. Description of the Related Art
Magnetic read sensors that utilize the giant magnetoresistive (GMR) effect for their operation are generally of the “current-in-the-plane” (CIP) configuration, wherein current is fed into the structure by leads that are laterally disposed to either side of an active sensor region and the current moves through the structure essentially within the planes of its magnetic and other conducting layers. Since the operation of GMR sensors depends on the detection of resistance variations in their active magnetic layers caused by changes in the relative directions of their magnetic moments, it is important that a substantial portion of the current passes through those layers so that their resistance variations can have a maximally detectable effect. Unfortunately, CIP GMR sensor configurations typically involve layer stacks comprising layers that are electrically conductive but not magnetically active and that play no role in providing resistance variations. As a result, portions of the current are shunted through regions that produce no detectable responses and, thereby, the overall sensitivity of the sensor is adversely affected. The CPP sensor configuration avoids this current shunting problem by disposing its conducting leads vertically above and below the active sensor stack, so that all of the current passes perpendicularly through all of the layers as it goes from the lower to the upper lead. The CPP configuration thereby holds the promise of being effective in reading magnetically recorded media having recording densities exceeding 100 Gbit/in2.
The pertinent prior art cited below has offered no similar method for improving the sensitivity of the CPP design having a synthetic spin valve stack configuration. Lederman et al. (U.S. Pat. No. 5,627,704) discloses a CPP GMR stack structure formed within a gap located in one of two pole layers of a magnetic yoke structure which also has a transducing gap formed in an ABS plane. The two pole pieces of the yoke serve to guide magnetic flux to the GMR stack which has current leads above and below it and permanent magnet biasing layers horizontally disposed on either side of it.
Dykes et al. (U.S. Pat. No. 5,668,688) discloses a spin valve CPP configuration in which the active layers form a stack of uniform width disposed between upper and lower shield and conductor layers.
Smith et al. (U.S. Pat. No. 6,473,279) teaches a CPP-GMR sensor whose ferromagnetic free layer is maintained in a single domain state by a layer configuration in which the free layer is separated from a pinning layer (below the free layer) by a non-magnetic spacer layer and an additional ferromagnetic layer is formed above the free layer and separated from it by an additional non-magnetic spacer layer formed of Ru. The Ru layer induces an anti-ferromagnetic exchange coupling between the additional ferromagnetic layer and the free layer and there is also a direct magnetostatic coupling between the additional ferromagnetic layer and the free layer. This combined interaction stabilizes the domain state of the free layer.
Redon et al. (U.S. Pat. No. 6,344,954) teaches a magneto-resistive tunnel junction whose ferromagnetic free layer and pinned layers are made of various layers of spin polarizing materials.
Nishimura (U.S. Pat. No. 6,226,197) teaches a magnetic thin film memory using a variety of ferromagnetic layered materials While the prior art cited above does make use of ferromagnetic materials like those to be used in the novel formation of the present invention, they do not address the issue of improving the sensitivity of a CPP device by a the formation of a free layer having improved magnetic characteristics. In particular, a good free layer for read head operation should be magnetically soft (have low coercivity) so that it can easily respond to external magnetic field fluctuations, yet it must also exhibit a small positive magnetostriction between 10−6 and 10−7 to reduce stress-induced magnetic anisotropy common in a free layer that is typically under compressive stress. The method of the present invention will produce such an improved free layer. In addition, the method of the present invention can also be advantageously applied to the formation of a synthetic antiferromagnetic pinned layer with improved characteristics. The cited prior art does not make reference to the improvement of CPP device performance by either such free layer or pinned layer improvement.